The present invention relates to a method and system for the remote condition monitoring of a structure, such as for example a welded structure in remote, inaccessible, and submerged locations and, is particularly suited to pipelines including submerged steel catenary risers.
Submerged or buried pipelines or other structures on locations such as mine sites, established chemical plants etc or in locations difficult to access, such as in nuclear power stations, or structurally sealed compartments in submarines present a problem in condition monitoring for the formation of cracking, which may arise due to vibration or dynamic loading. Often such cracking will initiate in a stress riser in the pipeline or structure, such as a weld or other joint. In the case of a pipeline, mobile internal pipe inspection equipment does not have the resolution to locate small flaws such as cracking of welds. Further, the use of such equipment often requires the shutdown of the pipeline in question. The monitoring of submarine compartments structurally sealed for long periods of time presents great difficulty for condition monitoring.
Also present known remote condition monitoring systems are often unreliable and difficult, if not impossible, to test. Accordingly it is at times hard to discern between the existence of an actual crack or a fault in the monitoring system itself.
It is an object of the present invention to provide a self-monitoring or self-testable method and system that facilitates remote condition monitoring of a structure to which it is applied, to give early warning of a flaw in a reliable and repeatable manner.
According to a first aspect of the present invention there is provided a self-monitoring or self-testable method for condition monitoring of a structure including the steps of:
forming one or more substantially sealed first cavities on or between surfaces of, or within said structure;
providing a fluid source of substantially constant pressure relative to a reference pressure where said constant pressure and said reference pressure are not the same;
coupling said fluid source to said first cavities through respective high fluid impedance devices sufficient to create detectable respective pressure differentials between said source and said first cavities across said high impedance devices;
providing a fluid capacitance in operative association with said first cavities and corresponding high fluid impedance devices to facilitate transient flow of fluid through said corresponding high fluid impedance devices; and,
providing a telemetry system operatively associated with said high impedance devices for monitoring transient or varying pressure differential across each high impedance device and, when a change in said pressure differential is monitored, providing a signal indicative of the location of the cavities coupled with said high impedance device across which said pressure differential is monitored.
Preferably said step of providing a telemetry system includes providing a plurality of differential pressure switches, each switch coupled across a respective high impedance device.
Preferably said step of providing said telemetry system further includes:
providing a fluid source pressure monitor switch having a first input in fluid communication with said fluid source and a second input in fluid communication with a said reference pressure, said fluid source pressure monitor switch arranged to change state when fluid pressure at its first input is substantially equal to the reference fluid pressure at its second input; and,
configuring said telemetry system to provide a signal indicative of a change in state of said fluid source pressure monitor switch.
Preferably said step of providing a telemetry system further includes providing a signal communication path between said differential pressure switches and said fluid source pressure monitor switch enabling said telemetry system to communicate with said switches.
Preferably said differential pressure switches are coupled in said signal communication path in a manner to enable parallel communication with said telemetry system.
Preferably said fluid source pressure monitor switch is coupled in series in said signal communication path.
In one embodiment said signal communication path is provided as a radio signal path.
In another embodiment, said signal communication path is provided as a plurality of transmission lines where said differential pressure switches are coupled parallel with each other across said transmission lines. Advantageously, said fluid source pressure monitor switch is coupled in series with said transmission lines in order to monitor the pressure condition of the fluid at an end remote from the source and confirm complete continuity of said transmission lines. In one variation, when said structure is made from an electrically conductive material, one of said transmission lines is comprised of said structure.
Preferably the step of providing said telemetry system further includes providing an AC signal generator for producing AC signals of different frequencies and transmitting said AC signals along said transmission lines; and
coupling a respective resonant trap or band pass filter in series with each differential pressure switch, said series resonance traps or band pass filters tuned to said different frequencies.
Preferably said method further includes coupling respective electrical chokes in parallel with each resonant trap or band pass filter.
In an alternate embodiment, said method includes the step of providing each switch with a unique address which is communicated via said transmission lines when a switch undergoes a change in state. In this embodiment said step of providing said telemetry system includes providing a processor based device in communication with said switches via said transmission lines for reading said addresses.
Preferably said step of providing said fluid source includes providing a gas at a substantially constant negative pressure relative to said reference pressure. When said reference pressure is ambient atmospheric pressure said negative pressure is a sub-atmospheric pressure. In this instance, advantageously said sub-atmospheric pressure is a vacuum. When said fluid source is a gas source said fluid capacitance is constituted by inherent elastic characteristic of gas provided by the source and the finite volume of at least said first cavities.
In an alternate embodiment, said step of providing said fluid source includes providing a liquid at a substantially constant pressure whilst the reference pressure is a liquid at a lower pressure.
In a further alternate embodiment, said step of providing said fluid source includes providing a liquid source at a substantially constant pressure whilst the reference pressure is a liquid at a higher pressure.
When using a liquid source said step of providing a fluid capacitance includes providing a pressure variable volume or simulated liquid capacitance such as an accumulator.
The provision of the fluid capacitance facilitates a self test or self monitoring behaviour that can be exploited by the sudden application of a fluid source of substantially constant pressure relative to a reference pressure, applied to the system wholly or separately to a particular cavity or cavities.
To facilitate the self-testing or self-monitoring feature, the method further includes the steps of:
temporarily coupling said first cavities to said reference pressure to substantially equalise fluid pressure in said first cavities and said high impedance devices with said reference pressure; and,
subsequently recoupling said fluid source to said first cavities to produce a transient fluid flow through said fluid capacitance and a consequential transient differential pressure sequentially across each high fluid impedance device thereby inducing sequential cyclic switching of said differential pressure switches.
Preferably said method further includes providing one or more second cavities in proximity to said first cavities and placing said second cavities in fluid communication with said reference pressure.
According to another aspect of the present invention, there is provided a self-monitoring or self-testable system for condition monitoring of a structure including at least:
one or more of substantially sealed first cavities formed on or between surfaces of or within said structure;
a source of fluid at substantially constant pressure relative to a reference pressure where said constant pressure and said reference pressure are not the same, said source coupled to said first cavities through respective high fluid impedance devices sufficient to create detectable pressure differentials between said source and said cavities across said high fluid impedance devices;
a fluid capacitance in operative association with said first cavities and corresponding high fluid flow impedance device to facilitate transient flow of fluid through said corresponding high fluid flow impedance devices; and,
a telemetry system operatively associated with said high fluid impedance devices for monitoring the transient or varying pressure differential across each high fluid impedance device and, when a change in pressure differential is monitored, providing a signal indicative of the location of the cavities coupled with the high fluid impedance device across which said pressure differential is monitored.
Preferably said telemetry system includes a plurality of differential pressure switches, each switch coupled across a respective high fluid impedance device.
Preferably said telemetry system further includes a fluid source pressure monitor switch having a first input in fluid communication with said fluid source and a second input in communication with said reference pressure, said fluid source pressure monitor switch arranged to change state when fluid pressure at its first input is substantially equal to fluid pressure at its second input, and wherein said telemetry system includes means for providing a signal indicative of a change in state of said fluid source switch.
Preferably said telemetry system includes a signal communication path between said differential pressure switches and said fluid source pressure monitor switch enabling said telemetry system to communicate with said switches.
Preferably said communication signal path provides parallel communication between said differential pressure switches and said telemetry system.
Preferably said fluid source pressure monitor switch is coupled in series in said signal communication path.
In one embodiment, said signal communication path is a radio signal path and said telemetry system includes a plurality of radio transceivers one provided at each of said differential pressure switches and said fluid source pressure monitor switch.
In an alternate embodiment, said signal communication path includes a plurality of transmission lines with which said differential pressure switches and said fluid source pressure monitor switch are connected.
In one embodiment, when said structure is made of an electrically conductive material, one of said transmission lines is comprised of said structure.
In one embodiment, said telemetry system further includes a plurality of resonant traps or band pass filters each tuned to different frequencies, respective ones of said resonant traps or band pass filters connected in a series with respective differential pressure switches.
Preferably said telemetry system further includes a plurality of electrical chokes, wherein individual chokes are coupled in parallel with respective resonant traps or band pass filters.
Preferably said telemetry system further includes an AC signal generator for producing AC signals of different frequencies for transmission to said differential pressure switches via said transmission lines.
In an alternate embodiment, each differential pressure switch is provided with a unique address which is communicated via said transmission lines when said switch undergoes a change in state. In this embodiment preferably said telemetry system includes a processor-based device in communication with said switches via said transmission lines for reading said addresses.
Preferably said fluid source includes a gas source at a constant negative pressure relative to said reference pressure.
In an alternate embodiment, said fluid source includes a source of a liquid at a substantially constant pressure relative to the pressure of a reference liquid. In this embodiment, said cavities includes first and second groups of sub cavities, said first group of sub cavities interleaves with said second group of sub cavities, said first group of sub cavities in fluid communication with said liquid source and said second group of sub cavities in fluid communication with said reference pressure.
Preferably, when said fluid is a liquid, said fluid capacitance includes respective pressure variable volumes, such as accumulators coupled in series with said high fluid impedance devices for transient displacement of fluid relative to the cavities coupled to said high fluid impedance devices and a further fluid capacitance or pressure variable volume in fluid communication with said fluid source at a location beyond a most distant high fluid impedance device to provide transient displacement with respect to said source.
Preferably, when said structure is in the form of a pipeline, said cavities are formed circumferentially on one or both of an outer circumferential surface and an inner circumferential surface of said pipeline. Further, when said pipeline is formed of a plurality of pipes joined together by circumferential welds, said cavities are formed over said circumferential welds.